Mixed ionic-electronic conducting ceramic membranes are of great interest for applications in catalytic reactors for the separation of oxygen and the conversion of hydrocarbons into added-value products, in particular the conversion of methane to syngas.
Catalytic membrane reactors, hereinafter called CMRs, produced from ceramics, allow the separation of oxygen from air, by making this oxygen diffuse in ionic form through the ceramic, and the chemical reaction of the latter with natural gas (principally methane) on catalytic sites (Ni or noble metal particles) deposited on the surface of the membrane.
However, ceramic membranes are by nature brittle materials which can withstand only very small deformations and have a very low ductility compared with metals. The various stresses exerted on a ceramic membrane are all the higher when the ceramic membrane is in a transient phase, i.e. in an off-equilibrium state between two steady states. Thus, for example, stresses develop in ceramic membranes within a CMR mainly during the start-up and shut-down phases.
The dimensions of a mixed ionic-electronic conducting ceramic membrane change when it is subjected to temperature changes. This is then referred to as thermal expansion.
Likewise, mixed ionic-electronic conducting ceramic membranes have the particular feature when they are subjected to a difference in oxygen partial pressure of letting O2− ions pass through them by a vacancy ion diffusion mechanism in the crystal lattice of the ceramic. The diffusion of oxygen or a modification in the concentration of oxygen vacancies in the material results in a deformation of the crystal unit cell, called chemical expansion.
In particular during the start-up phase of a mixed ionic-electronic conducting ceramic membrane, the chemical reduction (loss of oxygen) of the material constituting the membrane may cause very rapid chemical expansion capable of fracturing the membrane.
It is therefore imperative for the start-up phase of the mixed ionic-electronic conducting ceramic membrane to be controlled and carried out prudently.